1. Field of the Invention
The present invention relates to an optical sensor element of lateral pin structure diode type, and its drive method.
2. Related Art
In recent years, photodetection elements (such as photodiodes) using polycrystalline silicon or amorphous silicon formed as a film on a substrate by using the CVD method have been developed vigorously. A planar type illuminance detector is implemented by arranging photodiodes on a substrate. These photodiodes can be formed on a glass substrate or the like by diverting the technique of the thin film transistor (TFT) array used in, for example, liquid crystal displays.
In the typical conventional photodiodes, a reverse bias is applied in a vertical pin structure in which the pin structure is disposed perpendicular to the substrate or a lateral pin structure in which the pin structure is disposed in parallel to the substrate, and illuminance of light is detected by measuring a photocurrent that flows when the light is applied (see, for example, JP-A 2005-79438 (KOKAI)).
However, the vertical pin structure has a problem that the number of processes increases because it cannot be formed by using the same process as that of the TFTs for driving the photodiodes.
On the other hand, the lateral pin structure has a problem that the magnitude of the photocurrent becomes small although the structure can be formed in the same process as that of TFTs for driving the photodiodes. Furthermore, if the length of an i-region serving as a light receiving part of a photodiode in a direction sandwiched between a p-region and an n-region is lengthened, the value of the photocurrent varies among photodiodes having the same structure, resulting in another problem.